An adhesive that has the ability to establish multiple, discreet electrical connections between two electrical components is commonly referred to as an anisotropically conductive adhesive. Such adhesives often are used to provide electrical connection between a flexible circuit and an electrical substrate. An anisotropically conductive adhesive composition also should provide a short bond time, adhere to a variety of substrates, provide a bondline without voids, have satisfactory shelf and storage life, and maintain the physical connection between a flexible circuit and an electrical substrate. The anisotropically conductive adhesive composition should also be easy to manufacture and use.
Some anisotropically conductive adhesive compositions have used microencapsulated imidazoles as thermally activated curing agents. These anisotropically conductive adhesive compositions typically have a shelf life of approximately one week at room temperature. Such adhesive compositions are typically complicated to manufacture because solvent needs to be removed without initiating the cure by the imidazole curatives. If the solvent is not removed completely from such adhesive compositions, voids can result in the subsequent bonding operations. Voids in the bondline can reduce the reliability of the electrical connection during use and can also reduce the adhesion strength of the bonded electrical components. If solvent removal is protracted, the shelf life may be reduced due to partial release of the imidazole. If the viscosity in the adhesive composition prior to cure is too low, then voids may occur in the bondline. Increasing the viscosity of the uncured anisotropically conductive adhesive composition and/or using a curative that is operational at a lower temperature are known methods to reduce voids. However, higher viscosity formulations require more solvent and therefore longer steps to remove the solvent completely. Also, if the viscosity is too high, the coating solution may not wet the substrate, resulting in poor adhesion to the substrate. Lower temperature curatives can compromise both shelf stability and the manufacturing process.
Other types of anisotropic conductive adhesives with free-radical cure resins have been suggested in order to provide lower temperature and faster cures. One of the major weaknesses of these systems has been a lack of strong adhesion to a wide variety of flexible circuits.